Note (added 6/22/10): Variation B has been extensively
augmented to cover 11. cxb6, which soon leads to Black's
sacrifice of a rook.

Note (added 6/27/07): A discussion of some of the analysis
presented below may be found at Pete Tamburro's "Openings
for Amateurs" forum. I have been meaning to incorporate some
of that discussion into my analysis, but as I still haven't
done it, readers are encouraged to visit Pete's site (which
may require creation of a user login). Here is
the link to the applicable
thread.

Now to the analysis...

1. e4 Nf6 2. e5 Nd5 3. d4 d6 4. c4 Nb6 5. f4

The Four Pawns Attack of the Alekhine Defense, which
is believed to give White an advantage. But as Black's
first four moves seem quite sane, perhaps there is a
hidden resource for undermining the White pawn center
before it can be consolidated.

5...g5

This move has not done well in practice, but
I believe it is viable if followed up correctly.
Its purpose is to undermine White's pawn chain by
pressuring its base at f4, while enabling
Black's bishop to develop effectively at g7.
Though 5...g5 weakens the h6 square and the
b1/h7 diagonal, White's pawn advances seem at
least as compromising, having created holes at
d3, d4, e3 and e4. And as will be seen, the
published refutations are readily sidestepped
by Black.

Since many of the positions arising from 5...g5
are too complex to permit the enumeration of all
plausible alternatives, I will focus on what appear
to be the most critical lines.

Theory only gives 6...gxf4 7. dxc7 Qxc7 8. Nc3 which
is clearly better for White, given the
weakness of the f4-pawn.

7. fxg5

The pawn sacrifice has exposed White's d-pawn to
pressure along the a1/h8 diagonal, accentuated
by the tempo White has just expended. Moreover,
the extra pawn's being doubled means that White's
kingside pawn majority does not constitute a
potential passed pawn.

7...Bg7 8. Nc3 Nc6 9. Be3 d5 10. c5

10...e5

After 10...Nc4 11. Bxc4 dxc4 12. Nge2, Black seems
to have inadequate compensation for the pawn.

11. cxb6

More challenging than 11. dxe5 d4 12. cxb6 O-O, which leaves White with
many weak pawns.

11...exd4 12. Nb5 dxe3 13. Nc7+ Kf8 14. Nxa8 Bxb2 15. Rc1

The most viable means to prevent 15...Bc3+. At this point, 15...Bxc1
16. Qxc1 would seem to leave Black without an effective forcing continuation.